The present invention concerns transmitter and transceiver modules for the optical data transmission. These modules are in particular suited for the use in infra-red data transmission systems.
With the rapidly increasing number of workstations and personal computers (e.g. desktop or handheld ones) in all areas of business, administration, and fabrication, there is also an increasing demand for flexible and simple interconnection of these systems. There is a similar need as far as the hook-up and interconnection of peripheral devices, such as keyboards, computer mice, printers, plotters, scanners, displays etc., is concerned. The use of electrical wire networks and cables becomes a problem in particular with increasing density of systems and peripheral devices and in the many cases where the location of systems, or the configuration of subsystems, must be changed frequently. It is therefore desirable to use Wireless communication systems for interconnecting such devices and systems to eliminate the requirement of electrical cable networks.
In particular the use of optical signals for exchanging information between systems and remote devices received increased interest during recent years. The advantage of such wireless optical communications systems is the elimination of most of the conventional wiring. With respect to radio frequency (RF) wireless transmission, optical infrared (IR) wireless transmission has the advantages that no communication regulations apply and no PTT or FCC license is required. Additionally, no disturbance by electromagnetic interference and no interference from other RF channels can occur, and the radiation is confined to a room so that better data security is guaranteed than with RF systems. There is thus no interference with similar systems operating next door and a higher degree of data security is afforded than radio-frequency transmission can offer. In contrast to radio-frequency antenna, the dimensions of light emitting diodes (LED) and photodiodes are usually smaller, which is of particular interest when designing portable computers.
The optical signals in such systems might directly propagate to the optical receiver of the receiving system or they might indirectly reach the receivers after changes of the direction of propagation due to processes like reflections or scattering at surfaces. Today, the former case is realized in docking stations for portable computers where the data transfer takes place between an optical transmitter and a receiver which are properly aligned and close together at a distance on the scale of cm. The latter case is typical for applications in an office environment in which undisturbed direct transmission of optical signals between transmitters and receivers several meters away from each other is impractical or even impossible due to unavoidable perturbations of the direct path. One known approach to achieve a high degree of flexibility is to radiate optical signals from the transmitting system to the ceiling of an office where they are reflected or diffusely scattered. Thus, the radiation is distributed over a certain zone in the surroundings of the transmitter The distribution of the light signals spreading from the ceiling depends on many details which are characteristic for the particular environment under consideration. However, essential in this context is mainly that the transmission range, i.e. the distance between transmitting system and receiving system, is limited to some final value, hereafter called the transmission range, since the energy flux of the transmitted radiation decreases with increasing distance of propagation and the receiver sensitivity is limited due to a final signal-to-noise ratio. Typical known systems, operating at levels of optical power which are limited by the performance of the light sources and safety requirements for light exposure, have demonstrated transmission ranges of several meters for data rates of 1 Mbps.
Crucial parameters of a wireless optical communication system are the achievable data rate and the distance between the systems exchanging data. In an office environment, it can be necessary to communicate data over distances exceeding the transmission range of a conventional optical transmitter.
There are several disadvantages of todays wireless optical data transmission systems. First, the transmission range is not suited for use in environments such as for example large office rooms and conference rooms and the radiation characteristic and range is usually not uniform, thus requiring precise alignment of transmitter and receiver,
In addition, one has to take into account that in most environments there is unavoidable ambient light, such as daylight or light from lamps, which always reaches the optical detectors, unless the system is restricted for the use in a completely dark environment. Unavoidable ambient light can lead to time-dependent signals, for example AC signals from lamps, and is an important, in many practical cases the dominant source of noise in the optical receiver. Thus, ambient light influences the signal-to-noise ratio of the receiver and, therefore, affects the transmission range. The appearance of unavoidable light is mostly statistical and often difficult to control and its intensity can drastically change, as it is apparent for sunlight or lamps being switched on and off. A further realistic effect which statistically affects the signal-to-noise ratio and thus the transmission range is the occurrence of optical path obstructions influencing the receiver signal.
A first approach to get round these problems would be to increase the output power of the transmitter module. This has proven to be impractical for several reasons. The power consumption of such transmitter modules would be way to high for use in portable systems such as for example in notebook computers or palmtop computers. However, the most important issue facing the development of optical wireless systems is optical safety. It is anticipated that optical radiation can present a hazard to the eye and to the skin if the exposure is high enough. The degree of hazard depends on a number of factors, including the exposure level (energy or power), exposure time and wavelength.
In the article xe2x80x9cOptical Wireless: New Enabling Transmitter Technologiesxe2x80x9d, P. P Smyth et al., IEEE International Conference on Communications 93, May 23-26, 1993, Geneva, Switzerland, Technical Program, Conference Record, Volume ⅓, pp. 562-566, changes to existing eye safety standards as well as a new form of transmitter technology are discussed. This new form of transmitter technology is based on the idea to enlarge the area of the optical source in order to reduce the danger of retinal damage. In this article it is proposed to use a computer generated phase hologram for example, to obtain multiple beams for beam shaping out of a single laser diode source.
This approach is a first step in the right direction, but the problem of insufficient transmission range and sufficient eye-safety has not yet been addressed and solved.
It is an object of the present invention to provide for an improved optical transmitter module.
It is a further object of the present invention to provide for an optical transmitter module of small size and with optimum radiation pattern.
It is another object of the present invention to provide for an optical transmitter module which satisfies safety standards (IEC 825-1).
It is another object of the present invention to provide for an optical transmitter module with switchable radiation pattern.
The above objects have been accomplished by provision of optical transmitter modules as hereinafter claimed.